A graft copolymer is a polymer comprising molecules with one or more species of block connected to the main chain as side chains, having constitutional or configurational features that differ from those in the main chain, exclusive of the branch points. The simplest case of a graft copolymer can be represented by structure (I) ##STR1## where a sequence of A monomer units is referred to as the main chain or backbone, the sequence of B units is the side chain or graft, and X is the unit in the backbone to which the graft is attached. In graft copolymers, the backbone and side chains may both be homopolymeric, the backbone may be homopolymeric and the side chains copolymeric or vice versa, or both backbone and side chains may be copolymeric but of different chemical compositions. Branching in one or more stages and crosslinking may occur.
Graft polymerization is a common method for modifying polymer properties. Because the main chain and the branch chain are usually thermodynamically incompatible, most graft copolymers can be classified as multiphase polymers in the solid state. Free-radical polymerization methods are the oldest and most widely used procedures for the synthesis of graft polymers, because they are relatively simple.
Historically, graft copolymers have been prepared by polymerization of a monomer in the presence of a preformed backbone. The monomer can be polymerized by any of the traditional modes of polymerization. Backbones for free-radical graft copolymerization require the presence of an atom or group that can be abstracted or displaced by another radical, by radiation of sufficient intensity, or by mechanical degradation. Although free-radical graft copolymerization methods are the simplest, oldest, and most widely used, the least specific grafting sites and the most poorly defined branches result. Backbones for ionic or condensation polymerization require a reactive site or functional group capable of participating in specific chemical reactions. The products are well-defined and the properties of the branches can be controlled.
Graft copolymers can be prepared by copolymerizing preformed branches with the monomer constituting the major portion of the backbone. The branch must have an end group capable of copolymerizing with the monomer by the mode of polymerization used. Alternatively, preformed branches can be coupled with a backbone, if it has functional groups that can react with an end-group on the preformed branch.
Conventional techniques for graft copolymerizations use excessive amounts of toxic liquid solvents to dissolve the polymer, which in turn requires an expensive solvent recovery schemes in the process. Supercritical processing exploits non-toxic, non-flammable, and inexpensive carbon dioxide as a solvent which can be completely removed from the product by simply depressurizing the system.
In U.S. Pat. No. 4,748,220 to Hartmann et al., describes the preparation of pulverulent crosslinked copolymers in supercritical carbon dioxide under superatmospheric pressure in the presence of a free radical initiator. The monomer mixtures which were polymerized consisted of from (a) 70 to 99.99% by weight of monoethylenically unsaturated carboxylic acids, their amides and/or esters of such carboxylic acids and aminoalcohols, (b) from 0.001 to 10% by weight of a diethylenically or polyethylenically unsaturated monomer, and (c) from 0 to 20% by weight of other monoethylenically unsaturated monomers.
In U.S. Pat. No. 4,933,404 to Beckman et al., the use of supercritical fluids to polymerize monomers in a microemulsion system. This system comprises a first phase including a low-polarity material which is a gas at standard temperature and pressure, and which has a cloud-point density. It also includes a second phase including a polar fluid, typically water, a monomer (preferably soluble in the polar fluid), and a microemulsion promoter for facilitating the formation of micelles including the monomer in the system. The weight average molecular weight of the polymeric material polymerized at a temperature about the supercritical temperature of the fluid material is preferably at least 25%, more preferably at least 50%, and most preferably at least 100% greater than the weight average molecular weight of the polymeric material produced under substantially the same reaction conditions except that the polymerization is conducted at a temperature below the supercritical temperature of the fluid material.
In U.S. Pat. No. 4,990,595 to Traechkner et al., the melt viscosity of aromatic polycarbonates, aromatic polyester carbonates and aromatic and/or aliphatic polyesters in the molten state are treated with supercritical gases, is considerably lowered so that basic chemical operations which proceed only incompletely under the usual conditions can be carried out on these thermoplastics.